This invention relates to an apparatus and method for liquid-phase thermal diffusion isotope separation.
Liquid-phase thermal diffusion of isotopes in a molten single compound of the element being separated is known. This is evidenced, for example, by U.S. Pat. No. 2,968,403 to Abelson. Abelson discloses a thermal diffusion apparatus in which molten uranium hexafluoride is thermally diffused to separate .sup.235 U from .sup.238 U. The fluid is introduced into a narrow, elongate column between two vertical walls held at substantially different temperatures (64.degree. C. and 240.degree. C., respectively.) The lighter isotope is first driven to the hot wall by thermal diffusion, and then up by thermal convection currents driven by the temperature gradient across the slit.
The process disclosed by Abelson is useful only for liquids which are themselves a single compound of the element whose isotopes are to be separated, such as uranium hexafluoride. The process has also been used successfully to separate practical quantities of the isotopes of sulfur, chlorine, and bromine. Many elements, however, do not form stable, low molecular weight compounds which are liquid in a temperature range suitable for thermal diffusion separation. It is possible, in principle, to separate the isotopes of such elements using solutions of one of their solid compounds dissolved in a suitable solvent. Normally, however, the separation of solute from solvent when this is attempted is much greater than the separation of isotopic species. Abelson reports, for example, that aqueous solutions of potassium bromide subjected to thermal diffusion exhibit a concentration profile having a 22-fold relative change between ends of the column. In an efficient thermal diffusion column, nearly pure solvent accumulates at one end of the system and essentially all of the solute accumulates at the other. Unless counteracted, this tendency of the solvent and solute to separate reduces yields of the isotopic separation process to the vanishing point.
The goal, then, in using thermal diffusion to separate isotopes in solution is to counter solvent separation without creating parasitic circulations which remix the separated isotopes.
For non-isotopic systems, it has been demonstrated that the solvent-solute separation can be suppressed by imposing a net counterflow of solvent through the separation column. This is disclosed in H. Korsching, "Ein abgeandertes Verfahren bei der Trennung von Losungsbestandteilen durch Thermodiffusion in der Flussigkeit," Naturwissenschaften, 32, 220 (1944) and H. Korsching, "Ein neues Verfahren bei der Trennung von gelosten Stoffen durch Thermodiffusion in der Flussigkeit," Zeitschrift fur Naturforschung, 7b, 187 (1952). It can be shown that, in a short column, the solvent flow does not affect the separation of the components of the solute.
Isotopic separation using a counterflow technique as described above has been attempted and reported in an article written by the present inventor and another. W. M. Rutherford and K. W. Laughlin, "Separation of Calcium Isotopes by Liquid Phase Thermal Diffusion," Science, 211, 1054 (1981). The separations obtained by this process, as reported in the article, were too small to be of practical use. When attempts were made to obtain larger separations in longer devices, it was found that the fluid circulation in the slit becomes unstable, resulting in extensive remixing of the isotope material. Much of the isotopic separation that might have been expected to take place was thereby destroyed.